Columbium-tantalum base alloy



June l, 1965 E. F. DUFFEK, JR

COLUMBIUM-TANTALUM BASE ALLOY Filed Feb. 28. 1961 E R w 1. .w o o KR u 4 NCT T IMS T A S A E .L 4 T 4AS 4 O AOY w v RGw z H NHRL H o uA IoL /o SO/LE 0 5 OSAS 5 8 RSTA 5 R EB X 5 @WM M @www am x 4 N T H O3 U PN IHNL O T EO AOTC O2 Lo/N W ESOF T x 3 Reco E \||||I|||.||..I.|I|||| mmm/:Lm N .w F Zmm .m m DZDOm 2 EE LEM www P S ,ww/l1 e a o ..0 o 5 o 5 o 5 0 w 5 Q w 5 2 0 7 5 2 7 7 2 2 2 1 1 1 1 Ni BY WEIGHT INVENTOR EDWARD F. DUI-'FEA'` JR.

MTORNEYS..

United States Patent O 3,186,837 CLUivfmlUM-TANTALUM BASE ALLOY Edward F. Duifek, Jr., Mountain View, Calif., assignor to California Research Corporation, San Francisco, Calif.,

a corporation of Delaware Filed Feb. 28, 1961, Ser. No. 92,305 The portion of the term of the patent subsequent to Apr. 7, 1981, has been disclaimed 2 Claims. (Cl. 75-174) This invention relates to columbium base metal alloys, particularly to such alloys containing alloying metals of titanium, tantalum and nickel as essential components, which make the resulting alloys ductile and resistant to corrosion by hot oxidizing and hot nonoxidizing acids.

Alloys with the above properties are most useful for lining chemical reaction Vessels, auxiliary equipment and connecting lines, which are exposed to hot phosphoric acid and hot sulfuric acid. Although columbium has some resistance to concentrated acids at aboveV normal temperatures, severe corrosion and hydrogen embrittlement occurs as the temperature is raised to 400-450" F., especially in concentrated phosphoric acid.

Liquid phosphoric acid is recognized as an eflicient catalyst for certain organic chemical reactions, including .alkylation and isomerization and the polymerization of normally gaseous olefins. Corrosive attack appears to be the greatest with aqueous solutions of about 85% phosphoric acid. While considerable progress has been made in controlling corrosion in the presence of liquid phosphoric acid (see, for example, Kemp and Zeh Patent 2,653,177 and Piehl Patent 2,854,497), there is need for further improvement and particularly for alloys which are .also resistant to corrosion in the presence of hot oxidizing acids such as sulfuric acid. Sulfonations, cleavage and hydrolysis reactions often employ hot sulfuric acid. The most corrosive concentration for hot aqueous solutions of sulfuric acid appears to be about 55% sulfuric acid.

Prior art alloys are poor in corrosion resistance in the presence of either or both hot phosphoric acid and hot sulfuric acid. Usually, resistance to sulfuric acid corrosion is readily attained, but the characteristic of resistance to corrosion by hot phosphoric acid is difficult to add. Even more diflicult is to retain these corrosion resistant characteristics in an alloy that is ductile and can be worked sufficiently so" that it can be used without sole reliance upon casting as the method of shaping.

The metal alloy of this invention has the above desirable characteristics. It is a columbium base alloy comprised by Weight 2-15% titanium which, together with the other components, is needed to improve the workability and corrosion resistance of columbium. The amount of tantalum can vary widely from 20 to 50% but is preferably in the range of 30-40%. The fourth essential component is nickel in an amount suicient to give a two-phase alloy; usually about 2% is sufficient for this purpose. Up to 15 nickel may be incorporated but the higher nickel content alloys are more difficult to Work and are less resistant to corrosion by the more corrosive concentration of sulfuric acid. Nickel is preferably present in amounts of 3-5%.

Optionally, the columbium alloy may also contain up to 7.5% molybdenum, up to 7.5% tungsten and up to 10% vanadium, all of which form solid solutions in columbium. Also, up to 4% tin may be added. However, the alloyshould contain no more than about 15% total of these optional alloying elements. Often, at least 5% is desirable. Usually, only one of the four optional com- ICC ponents is added and that is preferably tungsten. The alloy should in any event contain at least 35% lcolumbium.

We have found that, as compared to high temperature oxidation in air, the corrosion under aqueous conditions such as the above aqueous acids is much more sensitive to impurities in the alloy. Hence, impurities inthe alloy should be kept to a minimum for the best results. Most desirably, the carbon content of the alloy should be no more than 0.03; the oxygen content, no more than about 0.05%; the nitrogen content, no more than 0.01%; the hydrogen content, no more than about 0.001%; the lead content, no more than about 0.001%. In addition, no more than about 0.3% of the combined elements aluminum, chromium, copper, iron, manganese, magnesium, silicon, zinc, zirconium, cerium and other rare earth metals should be present. Purity of the columbium component is most important. Thus, it is preferable to use electron-beam melted columbium since this is about 99.98% columbium. However, since most columbium is found associated with some tantalum which is desirable in the present alloy, less pure columbium can be employed, particularly if the contents of carbon, oxygen and nitrogen are low; thus, the columbium metal used in the alloy preparation should preferably contain no more than 0.01% carbon and no more than 200 p.p.m. of oxygen plus nitrogen. An analysis for a typically pure columbium is 0.021% carbon, 0.05% oxygen, 0.012% nitrogen, 0.001% hydrogen and 99.6% minimum of columbium.

The impurities are avoided by using pure components and inert atmospheres in the alloy preparations. Conventional melting and remelting techniques with inert controlled atmospheres of argon, helium and the like are employed. Arcl furnaces with water-cooled copper hearths and non-consumable electrodes (e.g., 2% thoriated-tungsten tip) were used in the preparations described below. Measured amounts of the metal components were melted in such furnaces. The alloy compositions, all in terms of Weight percent, were melted, the button of alloy inverted, and remelted four times before nal cooling and solidifying. Alternatively, other furnaces and known melting techniques, such as with continuous feed of metal components, inductive heating, or other means could be used with adequate protection against contamination.

The hardness of the resulting alloys depends upon the purity of the materials and the melting in inert controlled atmospheres. With these conditions and within the given ranges of composition, the resulting alloys are composited to give Rockwell hardness (C scale) in the range of 27 to 33. Solutionizing, by maintaining the alloy at 180C- 1900 F. for 24-72 hours, was used to homogenize and remove stresses in the as-cast specimens. This resulted in improving the workability of the alloy by reducing the hardness to the order of 20 (Rockwell C scale).

In the following illustrative examples, most of the alloys were clad in 310 stainless steel to avoid oxidation at the high temperatures used in fabrication. Cladding was done by lathe machining the alloy button into circular discs, enclosing them inside a 3 x 3 X 1t-inch steel sheet having a circular hole in the middle, and welding 2% x 2% x 1s-inch steel plates on each side. The resulting metal sandwiches of alloys were rolled at 2000 F. in a Stanat two-high mill, with reductions of 7.5-15% per pass to a total of about 70%. As indicated vin the examples, some of the metal alloys were cold Worked without prior cladding or metallurgical treatment. The machined as-cast buttons were cold rolled with intermediate annealing to a total cold reduction of about Reductions were about 10% for each pass through the rolling mill.

3 Y The alloys were tested for corrosion by exposing them to solutions of 55% sulfuric acid and 85% phosphoric acid sealed in heavy-wall Pyrex ampules. Alloyv specimens were prepared by grinding, sanding and finishing of surfaces and edges, and degreasing. Surface areas (generally about 6.5 cm), weights and densities were then determined. The density was measured by weightin weight-out of Water method. The volume of sulfuric and phosphoric acid solutions were 40 and 30 ml., respectively. Contaminations due to silica formation in the phosphoric acid tests were avoided by liners of Teflon tubing inside the glass ampules. The corrosion tests were for 72 hours. From the measurements of weight loss, density and surface area, the penetration or corrosion rates in mills per year (i.e., m.p.y.) were determined. The following table shows the results of tests carried out as described above.- Therein, the test labeled 85% H3PO4 at 400 F. in O2 was carried out by bubbling oxygen through the phosphoric acid before sealing the ampules. In the other corrosion tests, air was present above the acids in the ampules. The columbium used was that having a 99.6 minimum columbium content and for which the typical analysis of the impurities is given above.

respectively, and the nickel content is varied from to 5%, the rest being columbium. The graph illustrates that for service exposed .to hot sulfuric acid and to hot phosphoric acid, the nickel content should be at least sufficient to form two phases as shown by conventional microscopic examination of a polished section of the alloy. The upper limit preferred is governed by the exposure to hot sulfuric acid and is generally in the range of 4 to 5 percent, depending upon the purity of the columbium employed.

Other ductile columbium base 4alloys which illustrate the corrosion resistant (to hot sulfuric and phosphoric acids) alloy compositions of the present invention are tabulated as follows:

N Composition, percent by Weight Cb Ta Ti Ni Mo W V Sn COLUMBIUM BASE ALLOYS Compositions, alloy fabrication and corrosion resistance im sulfuric and phosphoric acid solutions Composition, percent by Weight Alloy fabrication Corrosion rates, m.p.y.

Cb Ta Ti Ni Mo W Sn Clad Solutionized Rolling 55% H2304 85% HaIOi 85% HEPO; at

temp., F. at 450 F. at 400 F. 400 F. in O3 100 Cold 209 67 33 Cold 22.6 64 33 Cold 23. 2- 33 11 11 2, 000 5l. 5 60 40 Cold 6. 4 49 40 10 1 1,900 F. for 70 hours Cold 4. 5 46 40 10 4 1,900 F. for 70 hours 1, 850 60 52 21 11 11 2,000 71. 4 95 Cold 85.8

*MgO used as a "parting compound between the 310 stainless steel cladding and the test alloy.

The above tests illustrate that while the basic binary compositions of columbium and tantalum were cold workable and had acceptable corrosion rates in sulfuric acid, the presence of nickel is necessary for satisfactory corrosion resistance on phosphoric acid.

Further increased resistance is obtained by using even higher purity columbium. Thus, when using electronbeam melted columbium, the following results were obtained.

Composition, percent Alloy fabrication by weight No. 55% H2804 85% HSPO.;

at 450 V. at 400 V. Cb Ta Ti Ni Clad Solutionized Rolling temp. V

10-.... 47 40 10 3 Yes. 1.900 F. for 65 hours 1, 950 40. 6 46. 7 11 46 40 l0 4 Yes.-. 1,900 F. for 65 hours 1, 950 61. 6 42. 0 12..-.. 45 40 10 5 Yes... 1,900 F. ior 65 hours... 1, 950 87. 3 45. 5 13"..- 50 40 10 Yes. 1, 850 6. 4 239 14..-..- 100 N0 Cold... 205k 1,250

Metallographic examination of the columbium alloys shows that the corrosion Vbehavior appears to depend partly on the phase composition. Thus, sufficient ynickel yields alloys with two phases which have very good resistance in hot phosphoric acid, but the resistance to sulfuric acid corrosion decreases with increasing nickel content. On the other hand, columbium alloys with up to 1% nickel were single phase in structure and possessed excellent resistance to sulfuricy acid but poor resistance to phosphoric acid.k This is illustrated in the accompanying graph, Where in the test compositions the amount of titanium and tantalum are held constant at 10 and 40%,

(References on following page) References Cited by the Examiner UNITED STATES PATENTS 6/26 Brace 75-174 2/58 Hix 75--174 12/ 60 Lyons 75--175.5 X 4/ 62 Wlodek `et al. 75-174 6/64 Wlodek 75-174 6 OTHER REFERENCES Sauer et a1.: Initial Investigation of Niobium and Niobium Base Alloys B.M.I., 1003, May 23, 1955, pages 1 and 37.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, WINSTON A. DOUGLAS,

. Examiners. 

1. A DUCTILE COLUMBIUM BASE ALLOY COMPOSITION RESISTANT TO THE ACTION OF HOT OXIDIZING AND NON-OXIDIZING ACIDS AND CONSISTING ESSENTIALLY OF IN WEIGHT PERCENT 2045% TANTALUM, 2-15% TITANIUM, A SUFFICIENT AMOUNT ABOVE 1% OF NICKLE TO GIVE A TWO-PHASE ALLOY BUT NO MORE THAN 15% NICKLE, UP TO 7.5% MOLYBDENUM, UP TO 7.5% TUNGSTEN, UP TO 10% VANADIUM AND UP TO 4% TIN, THE TOTAL OF MOLYBEDENUM, TUNGSTEN, VANADIUM AND TIN BEING NOT MORE THAN 15%, THE BALANCE BEING ESSENTIALLY COLUMBIUM WITH A PURITY OF AT LEASR 99.6%, SAID ALLOY CONTAINING AT LEAST 35% COLUMBINUM. 